1. Field of the Invention
This invention relates to a valve, and more particularly to a constant pressure valve.
2. Description of the Related Art
Referring to FIG. 1, a conventional constant pressure valve 10 is mounted into a manifold block 20. The manifold block 20 has a mounting hole 21 extending therethrough along an axial direction (X), a pressure-reducing port 22 connected fluidly to a workpiece (not shown), such as a hydraulic cylinder and a hydraulic motor, a fluid inlet 23 connected fluidly to a fluid supplying tank (not shown), and a pressure relief port 24 connected fluidly to a return fluid tank (not shown). The pressure-reducing port 22, the fluid inlet 23, and the pressure relief port 24 are in fluid communication with the mounting hole 21. In operation, fluid is fed through the fluid inlet 23 to flow into the pressure-reducing port 22 through the constant pressure valve 10. When the fluid pressure in the pressure-reducing port 22 exceeds a preset pressure, the fluid flows out through the pressure relief port 24 to maintain the pressure-reducing port 22 at the preset pressure, thereby facilitating steady operation of the workpiece.
With additional reference to FIGS. 2 and 3, the constant pressure valve 10 includes a lock rod 11 threaded into the mounting hole 21 in the manifold block 20, an inner fixed seat 12 secured in the lock rod 11, a valve sleeve 13 inserted fixedly into the lock rod 11 along the axial direction (X) and abutting against the inner fixed seat 12, a sliding shaft 14 movable axially in the valve sleeve 13, an outer fixed seat 15 secured in the valve sleeve 13 and in fluid communication with the pressure-reducing port 22, a compression spring 16 disposed between the sliding shaft 14 and the outer fixed seat 15, and a solenoid unit 17 threaded to the lock rod 11 along the axial direction (X).
The lock rod 11 has a first radial through hole 111 aligned with the pressure-reducing port 24, and a first inclined hole 112 in fluid communication with the pressure relief port 24.
The valve sleeve 13 has an inner end portion 131 disposed in the lock rod 11 and cooperating with the lock rod 11 to define an annular space 18 in fluid communication with the first radial through hole 111, an outer end portion 132 opposite to the inner end portion 131 and exposed from the lock rod 11, a second radial through hole 133 formed in the outer end portion 132 and in fluid communication with the fluid inlet 23, and a third radial through hole 134 formed in the inner end portion 131 and in fluid communication with the annular space 18.
The sliding shaft 14 has a large-diameter shaft section 140, an axially extending blind hole 141, a shoulder portion 142 defining an end of the blind hole 141, a small-diameter shaft section 143 extending from the shoulder portion 142 and through the inner fixed seat 12, fourth and fifth radial through holes 144, 145 disposed between the second radial through hole 133 and the third radial through hole 134, and a second inclined hole 146 extending through the shoulder portion 142. The outer diameter of the large-diameter shaft section is (D). The outer diameter of the small-diameter shaft section is (d).
The shoulder portion 142 of the sliding shaft 14 cooperates with the inner end portion 131 of the valve sleeve 13 and the inner fixed seat 12 to define a fluid storage space 19. The second inclined hole 146 is in fluid communication with the inner receiving chamber 141 and the fluid storage space 19.
The compression spring 16 biases the sliding shaft 14 to move rightwardly.
The solenoid unit 17 includes a fixed iron cylinder 171 threaded to the threaded section 313 of the lock rod 30, a movable iron rod 173 movable axially in an inner receiving chamber 172 in the fixed iron cylinder 171, and a coil member 174 disposed around the fixed iron cylinder 171. The movable iron rod 173 has a pushing rod section 175 movable to push the small-diameter shaft section 143 of the sliding shaft 14. The inner receiving chamber 172 in the fixed iron cylinder 171 is in fluid communication with the first inclined hole 112 in the lock rod 11.
When the coil member 174 is not energized, the input electrical current is 0 mA, a leftward pushing force applied by the pushing rod section 175 to the small-diameter shaft section 143 is 0 kg, and a rightward pushing force applied by the compression spring 16 to the sliding shaft 14 is k (elastic modulus)×L (pre-compressed length). In this state, the second radial through hole 133 of the valve sleeve 13 is misaligned from the fourth radial through hole 144 in the sliding shaft 14, so as to prevent fluid flow from the fluid inlet 23 into the sliding shaft 14. The third radial through hole 134 in the valve sleeve 13 cooperates with the fifth radial through hole 145 in the sliding shaft 14 to form a first opening 101 (see FIG. 2) therebetween. The fluid flows from the pressure-reducing port 22 into the inner receiving chamber 141 in the sliding shaft 14 via the compression spring 16, subsequently into the annular space 18 through the fifth radial through hole 145 and the third radial through hole 133, and eventually into the pressure-reducing port 24 and the return fluid tank through the first radial through hole 111 in the lock rod 11.
With particular reference to FIGS. 1 and 3, when an electrical current is input into the coil member 174, an electromagnetic force occurs between the fixed iron cylinder 171 and the movable iron rod 173 to provide a leftward pushing force to the small-diameter shaft section 143. As soon as the leftward pushing force is greater than the rightward pushing force applied by the compression spring 16 to the sliding shaft 14, the sliding shaft 14 is pushed by the movable iron shaft 173 to move leftwardly to thereby seal the first opening 101. At the same time, the fourth radial through hole 144 in the sliding shaft 14 is brought into fluid communication with the second radial through hole 133 in the valve sleeve 13 to form a second opening 102 therebetween. Hence, the fluid flows from the fluid inlet 23 into the inner receiving chamber 141 in the sliding shaft 14, and subsequently into the pressure-reducing port 22, the second inclined hole 146, and the fluid storage space 19, so that the fluid pressure in the pressure-reducing port 22 increases gradually, and is applied to the sliding rod 14. In accordance with Pascal's principle, all points in a sealed space have the same pressure. If the fluid pressure in the pressure-reducing port 22, the inner receiving chamber 141 in the sliding shaft 14, and the fluid storage space 19 is (P1), a rightward pushing force applied by the fluid in the pressure-reducing port 22 to the sliding shaft 14 is P1×πD2/4, and a rightward pushing force applied by the fluid in the fluid storage space 19 to the sliding shaft 14 is P1×[πD2/4−πd2/4]. Thus, the net rightward pushing force is P1×πd2/4. Since the electromagnetic force of the constant pressure valve 10 is F=k×L+P1×πd2/4, the pressure of the fluid in the pressure-reducing port 22 can be changed by adjusting the input electric current of the coil member 174. In addition, since the maximum value of the electromagnetic force F is fixed, to obtain a greater fluid pressure in the pressure-reducing port 22, it is necessary to reduce the outer diameter (d) of the small-diameter shaft section 143.
With particular reference to FIGS. 1 and 4, when fluid flows continuously from the fluid inlet 23 into the pressure-reducing port 22, the fluid pressure in the pressure-reducing port 22 increases. As soon as the rightward pushing force is greater than the leftward pushing force, the sliding shaft 14 is moved rightwardly. When the fluid pressure in the pressure-reducing port 22 increases, the first opening 101 (see FIG. 2) is still sealed, and the opening degree of the second opening 102 is reduced gradually. In accordance with Bernoulli's principle, the more quickly the fluid flows, the less the fluid pressure is. Hence, when the opening degree of the second opening 102 is reduced, the pressure drop between the fluid inlet 23 and the pressure-reducing port 22 is increased. Since the pressure in the fluid inlet 23 is fixed, the pressure (P1) in the pressure-reducing port 22 is reduced to the preset pressure.
Subsequently, with particular reference to FIGS. 1 and 5, the pressure (P1) in the pressure-reducing port 22 may exceed the preset pressure (even when the second opening 102 is closed) due to abnormal condition of the workpiece, e.g., a hydraulic cylinder is subjected to an external pushing force, or a hydraulic motor is subjected to a load. In this state, the sliding shaft 14 continues to move rightwardly until the first opening 101 is opened again. At this time, the fluid flows from the pressure-reducing port 22 into the pressure relief port 24 and the return fluid tank along a path including the first opening 101, the annular space 18, and the first radial hole 111. In this manner, the pressure (P1) in the pressure-reducing port 22 can be returned to the preset pressure.
FIGS. 2, 3, and 4 illustrate the pressure reduction function of the constant pressure valve 10. FIG. 5 illustrates the pressure relief function of the constant pressure valve 10. These two functions of the constant pressure valve 10 enable the pressure (P1) in the pressure-reducing port 22 to be maintained at the preset pressure. As described above, however, to obtain a greater fluid pressure of the pressure-reducing port 22, the outer diameter of the small-diameter shaft section 143 needs to be reduced. As a result, the strength of the small-diameter shaft section 143 is low, and thus is easy to deform or break when the small-diameter shaft section 143 is subjected to a high pressure.